Essential oils are a complex mixture of compounds that are produced by aromatic plants. They are evaporative lipophilic liquids with usually strong odour and with lower density than water. After isolation they are clear and colourless, rarely they become slightly coloured. Essential oils are used for health, spiritual and culinary purposes and their use dates back thousands of years in history. Despite the fact that the precise mechanisms of the essential oils are mostly unknown, aromatherapy is very spread today. In the master’s thesis we studied the chosen essential oils of the cypress family (Cupressaceae), using the method of gas chromatography together with mass spectrometry (GC-MS). We identified and quantified the compounds in essential oils. We were interested in the differences of composition in essential oils regarding the geographical origin, the species of the plant, and the plant part. The second part of the master’s thesis involved researching the activities of essential oils present in the analysed plants and individual compounds that were present in the analysed essential oils. Necessary data was obtained from the published in vitro and in vivo studies. From the data collected we tried to find links between the function of essential oils and their components. We analysed essential oils from seven different kinds of juniper and Mediterranean cypress. GC-MS analysis showed a complex composition of essential oils. The majority of essential oils contain two to three main compounds, which are present in high contents (more than 10%) compared to the majority of other compounds. The analysed essential oils had a very variable content of compounds based on their growing site, we also noticed the differences in the content of compounds between individual species. The essential oils of juniper and Mediterranean cypress have common antioxidant, anti-inflammatory, anticancer, antifungal and antimicrobial effects. The majority of research were made with essential oils of juniper species that proved to have antioxidant, anti-glycation, anti-inflammatory, anticancer, antifungal, antimicrobial, anti-hypercholesterolemic, anti-depressive, anxiolytic and anti-parasitic effects. They also have a beneficial effect on improving memory and treating symptoms of Alzheimer’s and Parkinson’s disease. We noticed a relatively good connection between the effects of essential oils and their individual compounds. The individual compounds have demonstrated many other biological effects that were not shown by the essential oils or the researchers have not studied them yet. We conclude, that based on the content of certain compounds in an essential oil we cannot deduce the effects of this essential oil. We can assume that the effects of essential oils aresynergetic effects of the individual compounds present in the essential oil.